Theory of Nb-Zr Alloy Superconductivity and First Experimental Demonstration for Superconducting Radio-Frequency Cavity Applications

TL;DR

This paper combines theoretical calculations and experimental validation to demonstrate that Nb-Zr alloys, especially with specific Zr concentrations, have enhanced superconducting properties and potential for improved SRF cavity performance.

Contribution

It provides the first experimental verification of high $T_c$ and reduced BCS resistance in Nb-Zr alloys, and shows Zr addition increases superheating field, advancing SRF technology.

Findings

Highest measured $T_c$ for Nb-Zr alloys to date

Reduced BCS resistance compared to conventional Nb

Increased superheating field $B_{sh}$ with Zr addition

Abstract

Niobium-zirconium (Nb-Zr) alloy is an old superconductor that is a promising new candidate for superconducting radio-frequency (SRF) cavity applications. Using density-functional and Eliashberg theories, we show that addition of Zr to a Nb surface in small concentrations increases the critical temperature $T_c$ and improves other superconducting properties. Furthermore, we calculate $T_c$ for Nb-Zr alloys across a broad range of Zr concentrations, showing good agreement with the literature for disordered alloys as well as the potential for significantly higher $T_c$ in ordered alloys near 75%Nb/25%Zr composition. We provide experimental verification on Nb-Zr alloy samples and SRF sample test cavities prepared with either physical vapor or our novel electrochemical deposition recipes. These samples have the highest measured $T_c$ of any Nb-Zr superconductor to date and indicate a reduction in BCS resistance compared to the conventional Nb reference sample; they represent the first steps along a new pathway to greatly enhanced SRF performance. Finally, we use Ginzburg-Landau theory to show that the addition of Zr to a Nb surface increases the superheating field $B_{sh}$, a key figure of merit for SRF which determines the maximum accelerating gradient at which cavities can operate.